Waist size matters more than BMI for heart risks

For the study, researchers analyzed data from 259,388 people who were part of 15 cohorts. The individuals either had waist circumference (WC) or waist-to-hip ratio (WHR) data and at least one of nine outcomes: time to first fatal and non-fatal myocardial infarction, fatal and non-fatal stroke, heart failure, atrial fibrillation, total coronary heart disease (CHD), total cardiovascular disease (CVD), CHD mortality, CVD mortality, and/or all-cause mortality.

The researchers used standard cutoffs for high WC (>88 cm for women and >102 cm for men) and high WHR (>0.85 for women and >0.90 for men). Those with BMIs between 18.5 and 24 were considered normal weight while those with BMIs between 25 and 29 were considered overweight and those with BMIs that were 30 or higher were considered obese.

For individuals with a normal weight, 5% had a large WC, and 18% had a high WHR. Among those who were overweight, 39% had a large WC and 40% had a high WHR. In comparison, among individuals who were obese, 9% had a small WC, and 45% had a low WHR.

For those with normal weight or overweight, a large WC and a high WHR were associated with a 15% to 50% excess risk for most heart disease outcomes over 20 years. Similarly, those who were obese had a 13% to 49% increased risk for heart disease outcomes associated with a large WC or high WHR. High WC was most strongly associated with heart failure and atrial fibrillation.

"WC and WHR identifies misclassification of conventional BMI categories and reclassifies CVD risk across normal weight, overweight, and obesity, adding diagnostic and prognostic value," they added. "We encourage clinicians to consider central adiposity distribution across the entire BMI spectrum when evaluating cardiovascular risk in primary prevention settings."

A growing push to move away from BMI

In recent years, there has been a growing effort from health experts and medical associations to move away from relying solely on BMI to assess a person's health and fitness.

Although BMI is commonly used to diagnose obesity, it cannot account for the distribution of body fat, which can contribute to health issues. Studies have shown that visceral fat, which surrounds the internal organs in the abdominal area, is associated with heart disease and diabetes.

"The frameworks are gaining conceptual traction faster than practical traction," said Brian Lee, a hepatologist and liver transplant specialist at Keck Medicine of USC. "Outside of research settings and some obesity medicine practices, anthropometric measures beyond BMI are still uncommon in routine visits."

According to Lee, the primary barrier to moving away from BMI isn't a lack of clinical understanding but a lack of time. Although BMI can be calculated automatically, clinicians have to manually measure WC or WHR.

New diagnostic frameworks could also significantly change who is classified as healthy or a normal weight. For example, a recent cross-sectional study found that 26.1% of people with a normal BMI met the criteria for "clinical obesity" under proposed guidelines that considered BMI and at least one other anthropometric measure (such as WC and WHR) or a direct fat measurement.

These changes could also have insurance implications, since treatment eligibility and coverage are often determined by BMI. Earlier this year, experts from the Endocrine Society warned that separating preclinical obesity from clinical obesity could delay care for millions of patients.

Overall, "[t]he goal isn't to discard BMI, which is useful and costs nothing," Lee said. "The goal is to not stop there."